The main goal of this design is to improve the ability to transmit and receive over a wide range of frequencies while still maintaining a low profile on the antenna. At the same time, this design can be used with standard feeders. The present invention belongs to antennas that transmit and receive electromagnetic oscillations with vertical polarization in the SW and USW bands and, more specifically, to antennas with low electric and geometric height.
Resonant Vertical Antennas
Resonant vertical antennas with height h≦0.1·λ electrically shorted with additional inductivity or capacity are widely used in practice. Antenna with additional inductivity provide quasi-sinusoidal current distribution along its length and the radiation resistance RS is defined as:
            R      S        =          20      ·                                    π            2                    ⁡                      (                          h              λ                        )                          2              ,where λ is wavelength of the operating frequency. Thus, creating a vertical antenna with height
      h    λ    =  0.01is not feasible because it has very low radiation resistance RS=2·10−2 OM and antenna efficiency of about zero.
Insertion of a capacity load into the upper part of the vertical antenna provides considerable increase in the current at the top of emitter and, in theory, can provide uniform current distribution along its length. With this alteration the radiation resistance is
      R    S    =      80    ·          π      2        ·                            (                      h            λ                    )                2            .      Further, the radiation resistance increases by a factor of four and for antenna with
      h    λ    =  0.01becomes RS=8·10−2OM. Though higher, that is also unacceptable for practical application. Moreover, these vertical antenna designs must have complicated elements to modify parameters of additional inductivity and a capacitor to provide frequency tuning.Slot Antennas
Slot antennas are widely used for microwave frequencies. They have a comparatively large input resistance RE=500˜1000 Ohm and hence high radiation resistance under efficiency η=0.7˜0.9. Inventor A. D. Blumlein received one of the first patents on the slot antenna design in 1938 (British Patent No 515684). A slot antenna is a narrow slot in a metallic plate. That slot has small height a in comparison to length 2l, equal to
            2      ·      l        =          v      ·              λ        2              ,where v is the contraction factor that depends on the permittivity ∈ of the medium in the slot and ratio of slot height a and length.
  v  =            f      ⁡              (                  ɛ          ,                      a                          2              ⁢              l                                      )              .  Generally slot length is about λ/2 for air dielectric when ∈≈1. Properties of slot radiation are the same as for a half-wave oscillator, yet there is an inverse distribution of magnetic and electric components in the field intensity.
In the case when slot antenna has dimensions 0.1·λ≦2·l<0.5·λ then the intrinsic impedance Zs of the slot at the middle of the curtain can be approximately estimated via parameters of an equivalent oscillator:
            Z      s        =                  2        ⁢                              (                          60              ·              π                        )                    2                            Z        d              ,where Zd=Rd+j·Xd is impedance of the equivalent oscillator. The active component of input impedance of the equivalent oscillator depends considerably on the slot size 2l:
            R      d        =                  ρ        d            ⁢                                    sh            ⁡                          (                              2                ⁢                kl                            )                                -                                    (                              γ                /                k                            )                        ·                          sin              ⁡                              (                                  2                  ⁢                                      kk                    1                                    ⁢                  l                                )                                                                          sh            ⁡                          (                              2                ⁢                kl                            )                                -                      cos            ⁡                          (                              2                ⁢                                  kk                  1                                ⁢                l                            )                                            ,            where      ⁢                          ⁢      k        =                  2        ⁢        π            λ      is a wavenumber; k1 is correction factor that takes into consideration influence of slot geometry on antenna contraction factor,
      γ    =                  R        s                              ρ          d                ·        l        ·                  [                      1            -                                          sin                ⁡                                  (                                      2                    ⁢                                          kk                      1                                        ⁢                    l                                    )                                                            2                ⁢                                  kk                  1                                ⁢                l                                              ]                      ;            R      s        =          80      ⁢                                    π            2                    ⁡                      (                          l              λ                        )                          2            is radiation resistance of the equivalent oscillator to the current loop;
      ρ    d    =      120    ⁡          [                        ln          ⁡                      (                                          2                ⁢                l                            r                        )                          -        1            ]      is oscillator impedance;
  r  =      a    4  is equivalent oscillator radius; and a is the slot height.
The reactive component of the equivalent oscillator resistance is:
      X    d    =            -      j        ⁢                  ⁢          ρ      d        ⁢                                        sin            ⁡                          (                              2                ⁢                                  kk                  1                                ⁢                l                            )                                +                                    (                              γ                k                            )                        ·                          sh              ⁡                              (                                  2                  ⁢                  kl                                )                                                                          ch            ⁡                          (                              2                ⁢                kl                            )                                -                      cos            ⁡                          (                              2                ⁢                                  kk                  1                                ⁢                l                            )                                          .      During this calculation it must be taken into account that the calculated reactive resistance of the equivalent oscillator is equal by value, yet of the opposite sign relative to the reactive component of input resistance of a non-resonant slot antenna. For instance, if the equivalent oscillator has a capacitive component of input resistance, then the equivalent slot antenna has an inductive component of input impedance. In other words, voltage distribution along the slot antenna corresponds as a first approximation to voltage distribution in line with shorted end so that input resistance at clamps on the middle of the long side of the slot antenna is of inductive nature.
For this reason, the resonant slot antennas with size
      2    ⁢    l    =      v    ⁢          λ      2      have some considerable disadvantages. First, they are narrow-band, therefore the tuning ratio is
            k      f        =                            f          max                          f          min                    ≤      1.1        ,where fmax is the maximum operation frequency and fmin is minimum operation frequency. Second, the uncompensated reactive component of input resistance is always present in the operation band. Finally, to excite a slot antenna at its center, it is necessary to apply a high-resistant feeder with non-standard impedance.